1. Field of the Invention
The present invention relates to data-over-cable systems and, more particularly, to a method and apparatus for facilitating downstream frequency override in data-over-cable system.
2. Description of the Background Art
With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems offer customers higher-speed connectivity to the Internet, an intranet, local area networks (“LANs”), and other computer networks via cable television networks.
The physical plant of the data-over-cable system may be divided into physically isolated branches that distribute digital signals from multiple cable modem termination system (“CMTS”) elements to geographically distinct groups of cable modems. Typically, a cable modem has access to downstream frequencies from more than one of the CMTS elements. By contrast, in the reverse or upstream direction, the cable modem has access to only one of the CMTS elements. Cable modems transmit to CMTS elements using a set of upstream transmission parameters that may differ from one CMTS element to another. A CMTS instructs cable modems to use particular upstream transmission parameters using messages carried in downstream transmissions.
When a cable modem is connected to a data-over-cable system, it must first gain access to the system by undergoing an initialization procedure before it is permitted to transmit data. A part of the initialization procedure includes a ranging process for adjusting the timing of the cable modem transmissions to its associated CMTS. This process involves sending a ranging message at an appropriate time interval, referred to as the Initial Maintenance interval. In some cases, a cable modem will synchronize to a downstream frequency provided by a CMTS other than the CMTS receiving the cable modem's upstream transmissions. When the cable modem sends a ranging message, the CMTS receiving the message re-directs the cable modem to use one of its downstream frequencies. This process is referred to as Modem Downstream Frequency Override (MDFO) or simply “frequency override.”
A cable modem sends the ranging message in accordance with upstream transmission parameters obtained from downstream transmissions of the CMTS with which it is synchronized. If the cable modem synchronizes to the wrong downstream frequency, however, these upstream transmission parameters may be different than those expected by the CMTS to which the cable modem sends upstream transmissions. As a result, the CMTS cannot process the ranging message and thus cannot perform the frequency override procedure. The CMTS will not be able to process any message from the cable modem, effectively preventing upstream transmission by the cable modem Accordingly, there exists a need in the art for facilitating downstream frequency override in a data-over-cable system.